(1) Field of the Invention
The present invention relates to a ballast tamping unit for mounting on a tamping machine adapted to tamp ballast under two successive ties fastened to two rails of a railroad track, the two ties defining a crib therebetween. The tamping unit comprises a vertically adjustable tamping tool carrier, a set of two pairs of opposed vibratory and reciprocable tamping tools mounted on the tamping tool carrier, the pairs of tamping tools being so spaced from each other in the direction of elongation of the track that two adjacent ones of the tamping tools of the pairs may be immersed in the crib between the successive ties and two remote ones of the tamping tools of the pairs may be immersed in respective cribs adjacent the successive ties, each tamping tool comprising a tamping pick having a substantially straight shaft having a center axis extending between a lower and an upper end, a tamping jaw at the lower tamping pick shaft end and a tamping pick holder, the upper end of the tamping pick shaft being detachably mounted in the tamping pick holder, and drive means connected to the tamping tools for vibrating and reciprocating the tamping tools. This invention also relates to a tamping tool for such a ballast tamping unit.
(2) Description of the Prior Art
Such ballast tamping units with twin tamping tools for track leveling, lining and tamping machines are known, for example, from U.S. Pat. Nos. 3,357,366, dated Dec. 12, 1967, No. 3,429,276, dated Feb. 25, 1969, and No. 4,773,333, dated Sept. 27, 1988. These twin tamping units have been commercially very successful because tamping machines incorporating the same are not only very efficient and productive because they enable two ties to be tamped simultaneously during each tamping stage but also because the two adjacent tamping tools immersed in the crib between the two successive ties are reciprocable in opposite directions to provide a very uniform ballast compaction under the two ties. However, tamping of ties separated by relatively narrow cribs or cribs of varying widths often causes problems since the shape of the tamping pick or its arrangement in the tamping pick holder, on the one hand, and the short reciprocating stroke limited by the narrowness of the crib reduces the degree of ballast compaction under the ties effectuated by the reciprocating tamping tools.
As the top view of FIG. 2 of U.S. Pat. No. 3,357,366 shows, the ballast tamping unit has two sets of two pairs of tamping tools arranged at respective sides of at least one of the track rails and along respective longitudinal edges of the two successive ties, the two sets at each track rail side having four adjacent tamping tools and the eight adjacent tamping tools of the four sets being reciprocable into an end position shown in FIG. 1 of the patent wherein the center axes and upper ends thereof substantially enclose an acute angle producing a wedge-shaped formation which allows only a limited reciprocating stroke of the tools in a narrow crib. To reduce this disadvantage, the linkage of the tamping tools is somewhat complex and the tamping jaws are transversely staggered so that the two adjacent tamping tools of each pair may be arranged as closely to each other as possible. This enables the tamping tools to penetrate more readily into encrusted ballast. When the crib widths are quite irregular or when obliquely positioned or double ties under rail abutments are encountered, it is not always possible to center the tamping tools quickly to avoid malfunctions. The outwardly tapering wedge-shaped formation of the two adjacent tamping tools in their one end position is particularly disadvantageous when they are immersed in narrow cribs since reciprocation of the two tamping tools towards the longitudinal edges of the two successive ties will cause the tamping pick shaft to impact on the edges, thus preventing further reciprocation and also damaging the ties.
In the tamping tool disclosed in U.S. Pat. No. 3,429,276, the tamping jaw is of corrugated configuration and the tamping pick shaft of the two adjacent tamping tools immersed in the crib between two successive ties is offset from the upper end of the tamping pick attached to the tamping pick holder. This design enables the tamping stroke of these two tamping tools to be increased so that the machine works satisfactorily in cribs of average width. Therefore, the twin tampers incorporating such tamping tools have had great commercial success but the offset structure of the tamping picks makes their manufacture more complicated than tamping picks with straight shafts. In addition, these tamping tools still fail to permit a sufficient tamping stroke in very narrow cribs.
The ballast tamping unit disclosed in U.S. Pat. No. 4,404,913, dated Sept. 20, 1983, comprises twin pairs of tamping tools so spaced that each pair is immersible in a respective one of two successive cribs, and the tamping tools of each pair are reciprocable towards the two adjacent ties defining each crib so that, during each tamping stage, the tie between the two pairs of tamping tools is fully tamped while only one half of each of the other ties defining the two successive cribs is tamped. In their initial end position upon immersion in the ballast of each crib, the two reciprocable tamping tools of each pair define a wedge-shaped formation and, as explained hereinabove in connection with U.S. Pat. No. 3,357,366, this limits the reciprocating stroke of the tamping tools.